Replaces feature geometry with a convex or concave hull around the feature.

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Typical Uses

  • Generating hulls around individual features

How does it work?

The HullReplacer receives features with any geometry type and individually replaces their geometry either a Convex or Concave hull.

A convex hull is a polygon covering the extents of the feature, where no interior angle is greater than 180 degrees. It resembles an elastic band surrounding the feature.

A concave hull (or alpha hull) has no restrictions on angle and may follow the general shape of the feature more tightly. It is calculated using the Alpha Value parameter and can have multiple parts and donuts.

If an output hull has zero area (the hull of a single point or vertical line, for example), it will become a point or line as appropriate.

3D hulls may be generated when input features are either 3D or a mix of 3D and 2D. If mixed, 2D features are assigned a default Z value.

Concave hulls and alpha values

Concave hulls vary in shape. They are calculated by first triangulating the dataset, then measuring the circumradius of the resulting triangles (the radius of a circle that touches all three points of the triangle). Triangles that are larger than the specified Alpha Value are discarded, and the rest are dissolved to form the hull. Measurements are in ground units.

If the Alpha Value is explicitly specified, it is used for the calculation. If it is zero (0) or not specified, the alpha is approximated as the smallest value that produces a single area. If the alpha is too small it may return a null geometry. Larger numbers generally produce larger areas, and a large enough alpha value produces the same result as a convex hull.

See Examples below to see the effect of varying the Alpha Value.

Geometries that are farther away than Alpha Value from the rest of the geometries may be discarded if they do not constitute a valid area on their own.

Concave hull generation works best with points, but line and area geometries will be accepted and converted to points if input.


Usage Notes

  • Point clouds are considered single features. To create a hull from a point cloud’s individual points, use a PointCloudToPointCoercer beforehand, setting the Output Geometry parameter to either Single Multipoint or Individual Points.
  • For triangulated surfaces, see the SurfaceModeller, Triangles output.


Input Ports

Output Ports


Editing Transformer Parameters

Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click beside the applicable parameter. For more information, see Transformer Parameter Menu Options.

Defining Values

There are several ways to define a value for use in a Transformer. The simplest is to simply type in a value or string, which can include functions of various types such as attribute references, math and string functions, and workspace parameters. There are a number of tools and shortcuts that can assist in constructing values, generally available from the drop-down context menu adjacent to the value field.

Dialog Options - Tables

Transformers with table-style parameters have additional tools for populating and manipulating values.


Processing Behavior


Feature Holding


Dependencies None
Aliases ConvexHullCreator

FME Community

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Search for all results about the HullReplacer on the FME Community.


Examples may contain information licensed under the Open Government Licence – Vancouver and/or the Open Government Licence – Canada.